As an external modulator, Mach-Zehnder (MZ) type modulators such as LiNbO3 modulators (LN modulator) are known in which the optical transparency characteristic changes to a sinusoidal waveform (cycle 2Vπ) with respect to applied voltage. However, this transparency characteristic is sensitive to disturbances such as temperature changes, and hence temporal changes (operating point drift) occur therein.
Here, “operating point drift” refers to a drift in a direction in which applied voltage increases or decreases, in the characteristic curve representing a relationship between applied voltage and optical output power.
Among various methods for compensating for operating point drift of an external modulator such as a Mach-Zehnder (MZ) type modulator, a low-frequency superimposing method has been widely used because of its simplicity and convenience. This low-frequency superimposing method is a method in which a low-frequency signal is preliminarily superimposed on a modulation signal, and the amount of an operating point shift is compensated for according to a phase difference between the low-frequency signal component detected in the output of the external modulator, and the original low-frequency signal. FIG. 10 shows a configuration of a conventional control apparatus 21 for an external modulator that uses of the low-frequency superimposing method (Patent Document 1).
In FIG. 10, the conventional control apparatus for an external modulator to be used in optical communication is provided with a light source 20, a Mach-Zehnder (MZ) type optical modulator 22, an optical branching unit 23, a low-frequency oscillator 24, a low-frequency superimposing unit 25, a low-frequency signal detecting unit 26, and an operating point control unit 27.
The optical branching unit 23 branches and outputs the optical output from the Mach-Zehnder (MZ) type optical modulator 22 to an optical output unit (not shown in the figure) side and to the low-frequency signal detecting unit 26 side.
The low-frequency oscillator 24 generates and outputs low-frequency signals (frequency f) for detecting the amount of operating point shift of the Mach-Zehnder type optical modulator 22.
The low-frequency superimposing unit 25 superimposes a modulation signal for modulating light output from the light source 20, and a low-frequency signal (frequency output from the low-frequency oscillator 24.
The low-frequency signal detecting unit 26 detects, among the optical outputs of the Mach-Zehnder (MZ) type optical modulator 22, the low-frequency signal component (frequency f) that has been superimposed on the modulation signal by the low-frequency superimposing unit 25. Moreover, it outputs a direct current voltage signal according to the phase difference (the amount of the operating point shift of the Mach-Zehnder type optical modulator 22) between this low-frequency signal component (frequency f) and the low-frequency signal (frequency f) output from the low-frequency oscillator.
The operating point control unit 27 controls the operating point based on the electrical signal output from the low-frequency signal detecting unit 26, so as to compensate for the amount of operating point shift (operating point drift) of the Mach-Zehnder (MZ) type optical modulator 22.
In the above configuration, the continuous light output from the light source 20 is input to the Mach-Zehnder type optical modulator 22, and optical modulation is performed. The electrical signal used in this optical modulation is a signal in which the low-frequency signal (frequency f) output from the low-frequency oscillator 24 has been superimposed on the modulation signal by the low-frequency superimposing unit 25. Therefore, the optical modulation signal contains the low-frequency signal (frequency f) component.
In the Mach-Zehnder type optical modulator 22, the continuous light output from the light source 20 is modulated by means of the modulation signal containing the low-frequency signal component output from the low-frequency superimposing unit 25, and is then output to the optical branching unit 23.
The modulation output of the Mach-Zehnder type optical modulator 22 is branched and output by the optical branching unit 23, to the optical output unit side and the low-frequency signal detecting unit 26 side.
The low-frequency signal detecting unit 26 detects the low-frequency signal component (frequency f), compares the phase of the detected low-frequency signal with the phase of the low-frequency signal (frequency f) output from the low-frequency oscillator 24, and outputs, to the operating point control unit 27, phase difference information, that is, a signal that signifies the amount of operating point shift of the Mach-Zehnder type optical modulator 22 (direct current voltage signal).
In this low-frequency detecting unit 26, the optical output that has been taken out at the optical branching unit 23 is converted by a photodiode into an electrical signal to be detected. This electrical signal contains the low-frequency signal that has been superimposed in the low-frequency superimposing unit. After being appropriately amplified in an amplifier, this detected electrical signal is mixed by a mixer with the low-frequency input from the low-frequency oscillator to the mixer, and the phase is detected. In the output from the mixer corresponding to this phase, signals above a predetermined frequency are cut off by a low-pass filter, and a direct current voltage signal is output to the operating point control unit 27.
In the operating point control unit 27, a control voltage for compensating for the operating point drift (operating point shift amount) of the Mach-Zehnder type optical modulator 22 based on the phase difference information, is applied to the Mach-Zehnder type optical modulator 22. That is to say, a bias voltage is shifted according to the electrical signal output from the low-frequency signal detecting unit 26, and the operating point drift of the Mach-Zehnder (MZ) type optical modulator 12 is compensated.
By means of a feedback control operation with a feedback loop of the above optical branching unit 23, the low-frequency signal detecting unit 26, and the operating point control unit 27, it is possible to compensate for the operating point drift of the Mach-Zehnder type optical modulator 22 and to stably perform modulation, following the operating point drift.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H03-251815